A drive mechanism (11) for a delivery device comprising a drive member (17) for driving a piston rod (13); a moveable dose setting member (19, 35) being disengaged from the drive member (17) in a dose setting state and being moveable in a drive direction in a dose delivery state, wherein the dose setting member (19, 35) engages with the drive member (17) in the dose delivery state in such a manner that the movement of the dose setting member (19, 35) in the drive direction is transferred to the drive member (17); and a spring member (21) being coupled to the dose setting member (19, 35) in such a manner that the movement of the dose setting member (19, 35) in a setting direction loads the spring member (21) in the dose setting state, the spring force of the loaded spring member (21) driving the dose setting member (19, 35) in the drive direction in the dose delivery state.